"Indurate" is a hard word — in more than one way. Not only is it fairly uncommon in modern usage, but it also can be traced back to Latin "durare," meaning "to harden." (“Durare” can mean "to endure" as well, and appropriately “indurate” is a word that has lasted many years — it has been a part of the English language since the 14th century.) "Durare" is also the root of other durable English words, including "during," "endure," "duration," "durance" (an archaic word meaning "endurance"), and even "durable" itself. In addition, "indurate" can be a verb meaning "to make or grow hard," "to make unfeeling, stubborn, or obdurate," and "to establish firmly."